According to the bar's website, a "College Country" night was going on at the time of the shooting.
Multiple people were wounded after a gunman fired multiple shots in Thousand Oaks. Police were apparently advised that the suspect was a Middle Eastern man in his early 20s with a beard and wearing all black clothing.
Dozens of police cars and emergency service vehicles were spotted at the scene in Thousand Oaks around 11.20pm local time on Wednesday.
Social media reports claim the suspect shot a security guard before entering the building and throwing smoke grenades before firing more shots.
The FBI are now at the scene of the shooting and the bomb squad has been requested. They can not yet confirm if the suspect is on the loose.
Ventura County Sheriff's Capt. Garo Kuredjian said hundreds of people were inside the bar at the time of the shooting.
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He told reporters when officers arrived at the bar to reports of shots being fired, they could hear gunfire.
It has not yet been revealed the extent of the injuries.
It is reported to be "college night" at the bar, with some suggesting the attack was aimed at university students.
Authorities urged the public to avoid the area.
Police officers were called to Borderline Bar & Grill in Thousand Oaks at around 11:20 p.m., The Ventura County Star reported.
Witnesses who were at the bar are describing a scene of panic inside, and say people reportedly used chairs to break windows to escape.